DiGeorge syndrome (DGS) is a developmental anomaly of the derivatives of the 3rd and 4th pharyngeal pouches. It results in a variety of malformations including absence of hypoplasia of thymus and parathyroids, cardiovascular anomalies and mild craniofacial dysmorphia.
It has been proposed that the primary defect in DGS is due to the failure of cephalic neural crest cells to migrate properly during early embryonic development (Kirby et al., 1984; Lammer and Opitz). Previously, cytogenetic studies of patients with DGS demonstrated that .about.20% had chromosomal abnormalities, with the majority of these chromosomal rearrangements involving the loss of the proximal long arm of chromosome 22 (rev. Greenberg, 1988). These results suggested that monosomy for 22q11 may play a significant role in the etiology of DGS. Subsequently, molecular studies have demonstrated the validity of this hypothesis (Driscoll et al., 1992a; Carey et al., 1993) and microdeletions have been detected in 89% of the patients we have studied with DGS (Driscoll et al., 1994).
Velo-cardio-facial syndrome (VCFS) is a common autosomal dominant disorder characterized by cleft palate, cardiac anomalies, typical facies and learning disabilities. Due to the phenotypic overlap between VCFS and DGS it was suggested that both diseases might share a common pathogenesis or be etiologically related (Stevens et al., 1990). Using the 22q11.2 markers found to be hemizygous in DGS, it was possible to demonstrate that VCFS patients are deleted for the same region (Driscoll et al., 1992b). Currently, over 85% of the carefully selected patients we have studied with a diagnosis of VCFS have microdeletions of 22q11.2. These findings indicate that haploinsufficiency of this region is also a major factor in the etiology of this disorder (Driscoll et al., 1994).
The region commonly deleted in the majority of DGS/VCFS patients has been estimated to be greater than 1.4 Mb, based on pulsed-field gel analysis. Further, the region deleted in individual patients can extend both proximally and distally (Drisoll et al., 1992a).
Using translocation breakpoints and fluorescence in situ hybridization analysis (FISH), the DGS/VCFS chromosomal region (DGCR) has been narrowed to 250 kb in the vicinity of D22S75 (N25) (Li et al., 1994). The only known balanced translocation associated with the DGS/VCFS phenotype, the ADU/VDU t(2:22), maps within this 250 kb region (Augusseau et al., 1986; Budarf et al., 1995). These data suggest that one or more of the genes in this minimal DGS/VCFS chromosomal region (mDGCR) are strong candidates for involvement in the pathogenesis of these disorders.
The construction of a detailed transcription map covering the complete DGCR, with emphasis on the minimal critical region, is an essential step in the identification of genes important to the etiology DGS/VCFS. In the present study, we have identified genes encoded in the mDGCR using a cDNA selection-based approach. Concurrent with the construction of this transcription map, large-scale genomic sequence of the cosmids covering the mDGCR has been performed (Roe et al., in preparation).
The availability of genomic sequence permitted the unambiguous verification of transcripts, the determination of the direction of transcription and the identification of intron-exon structure. Further, the use of GRAIL and BLAST programs assisted in the assembly of the cDNA clones into transcription units. Here we report the identification, now, of at least 12 genes in the 250 kb of the mDGCR by this combined cDNA selection and DNA sequencing approach. These genes can be considered candidates for the major features of DGS/VCFS.